1. Field of the Invention
The present invention relates to a distortion estimation apparatus, frequency offset compensation apparatus, reception apparatus and their method used in a radio communication apparatus.
2. Description of the Related Art
In radio communications for which the demand has been increased rapidly in recent years, the improvement of spectral efficiency is required. One of effective technologies for improving the spectral efficiency is a QAM (Quadrature Amplitude Modulation).
In order to apply the QAM to the radio communication, it is necessary to compensate a fading distortion of a transmission signal generated under the Rayleigh fading environment.
As a method, conventionally known, for compensating such a fading distortion, there is, for example, xe2x80x9cRayleigh Fading Compensation Method for 16QAM MODEM in Digital Land Mobile Radio Systemsxe2x80x9d (SAMPEI, Technical Report of IEICE B-II, Vol. J-72-B-II, No. 1, pp. 7-15, January, 1989).
This method relates a system in which a transmission side transmits transmission data with a frame structure comprised of the predetermined number of symbols with a pilot symbol, known between the transmission and reception sides, inserted between data symbol sequences periodically, and the reception side detects a signal space position of the pilot symbol, and estimates a fading distortion amount of each symbol from the relationship between the relative position of each point to compensate.
This method uses the characteristic of the radio communication that the scale and gradient of a signal space diagram varies depending on an amplitude variation and phase variation of the transmission signal generated under the rayleigh fading environment, however the entire shape of the signal space diagram does not vary.
An example of conventional technologies using this system is explained below using accompanying drawings.
FIG. 1 is a block diagram illustrating a configuration of a conventional distortion compensation apparatus.
The distortion compensation apparatus in FIG. 1 is mainly comprised of clock reproduction section 11, pilot detection section 12, pilot symbol extraction section 13, distortion amount calculation section 14, data symbol extraction section 15, data delay section 16 and distortion estimation compensation section 17.
In the distortion compensation apparatus in FIG. 1, a received complex baseband signal u(t) is inputted to clock reproduction section 11, pilot detection section 12, pilot symbol extraction section 13, and data symbol extraction section 15.
Clock reproduction section 11 controls a clock signal oscillated from an internal oscillator based on the received complex baseband signal u(t), and outputs the controlled clock signal to pilot detection section 12 and data symbol extraction section 15.
Pilot detection section 12 detects a position of the pilot symbol in the received complex baseband signal u(t) based on the clock signal, and outputs a signal indicative of the position of the pilot symbol to pilot symbol extraction section 13.
Pilot symbol extraction section 13 extracts the pilot symbol from the received complex baseband signal u(t) at the same timing when a signal is inputted from pilot detection section 12, and outputs the extracted pilot symbol to distortion amount calculation section 14.
Herein, when it is assumed that a transmission complex baseband signal is z(t), and that a fading distortion is c(t), the received complex baseband signal u(t) can be expressed with the equation (1) shown below.
U(t)=z(t)xc3x97c(t)xe2x80x83xe2x80x83(1)
Further, when it is assumed that a received pilot symbol is up, and that a transmission pilot symbol is tp, a fading distortion amount cp of the pilot symbol can be obtained with the equation (2) shown below.
cp=up/tpxe2x80x83xe2x80x83(2)
Distortion amount calculation section 14 calculates fading distortion amount cp of the pilot symbol with the above equation (2) to output to distortion estimation compensation section 17.
Data symbol extraction section 15 extracts a data symbol from the received complex baseband signal u(t) based on the clock signal to output to data delay section 16.
Data delay section 16 delays the inputted data symbol by a time required in distortion estimation compensation section 17 for estimating the fading distortion amount of the data symbol to output to fading distortion estimation compensation section 17.
Distortion estimation compensation section 17 interpolates the fading distortion amount of the data symbol on the time axis based on fading distortion amount cp of the pilot symbol to estimate. Then, distortion estimation compensation section 17 multiplies the data symbol by a reciprocal (complex conjugate) of the fading distortion amount of the data symbol to compensate the fading distortion of the data symbol.
FIG. 2 is a diagram showing a relationship between time (abscissa) and distortion amount of each symbol (ordinate). As illustrated in FIG. 2, approximate polynomial 21 is determined based on the fading distortion amount of the pilot symbol (∘), and fading distortion amount of each data symbol (xe2x97xaf) is placed on approximate polynomial 21. In addition, FIG. 2 adopts the frame structure in which the pilot symbol is inserted periodically every 8 symbols.
Thus, the above-described conventional distortion apparatus performs the interpolation based on the fading distortion amount of the pilot symbol, and thereby estimates the fading distortion amount of the data symbol to compensate.
As the causes of the distortion generated in the transmission signal in radio communications, there is a frequency offset between a transmitter and receiver besides the fading.
However, in the above-described conventional distortion compensation apparatus, since the frequency offset is not considered, the estimation accuracy lowers in the case where the frequency offset is large, resulting in the problem that the bit error rate characteristic deteriorates.
A first object of the present invention is to estimate a frequency offset and fading distortion with high accuracy even in the case where the frequency offset between a transmitter and receiver is large, and to suppress the deterioration of the bit error rate characteristic.
The above object of the present invention is achieved by calculating an amplitude variation amount of data symbol and phase variation mount of the data symbol separately, or estimating a fading distortion of the data symbol using the pilot symbol subjected to frequency offset compensation.
Further, a second object of the present invention is to obtain excellent bit error rate characteristic even in the case where the fading variation is large, and a signal with low received field strength is received.
The above object of the present invention is achieved by judging whether or not the pilot symbol was detected effectively, and calculating the frequency offset using only the effectively detected pilot symbol.